Inspection method of electrical storage device and manufacturing method thereof

ABSTRACT

An inspection method of an electrical storage device includes: setting a pseudo parasitic resistance value to be small in a case where a power storage capacity of the electrical storage device is large, while setting the pseudo parasitic resistance value to be large in the case where the power storage capacity is small; in a state where the pseudo parasitic resistance value is set, acquiring a current value after convergence of a current flowing through a circuit such that the circuit is formed by connecting an external power source to the charged electrical storage device in a direction where a voltage is applied thereto; and determining quality of the electrical storage device based on the current value.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2017-248113 filed onDec. 25, 2017 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The disclosure relates to an inspection method for determining qualityof an electrical storage device. More specifically, the disclosurerelates to an inspection method for an electrical storage device, theinspection method being able to quickly perform quality determinationbased on a discharge current amount, not based on a voltage drop amountof the electrical storage device. The disclosure also relates to amanufacturing method of an electrical storage device, the manufacturingmethod including the inspection method of the electrical storage deviceas part of steps.

2. Description of Related Art

Various inspection methods have been conventionally proposed as aninspection method for determining quality of electrical storage devicessuch as a secondary battery. For example, in Japanese Unexamined PatentApplication Publication No. 2010-153275 (JP 2010-153275 A), a leavingstep of leaving a secondary battery as a determination target to standin a pressurized state is performed. A battery voltage is measuredbefore and after the leaving step. A difference between the batteryvoltages measured before and after the leaving step is a voltage dropamount along with the leaving. A battery with a large voltage dropamount has a large self-discharge amount. On this account, it ispossible to determine quality of the secondary battery based on amagnitude of the voltage drop amount. Such an inspection method may beperformed as one step in a manufacturing method.

SUMMARY

However, the quality determination of the secondary battery in therelated art has the following problems. That is, the qualitydetermination takes time. The reason why the quality determination takestime is because the leaving step should take a long leaving time toobtain a significant voltage drop amount. One factor thereof is acontact resistance at the time of voltage measurement. The voltagemeasurement is performed by connecting a measuring instrument betweenterminals of the secondary battery. At this time, a contact resistanceis inevitably caused between the terminal on the secondary battery sideand a terminal on the measuring instrument side, so that the measurementresult is affected by the contact resistance. The contact resistancevaries every time the terminal on the secondary battery side isconnected to the terminal on the measuring instrument side. On thisaccount, if the voltage drop amount itself is not large to some extent,variations in contact resistance between measurements cannot be ignored.

Further, the accuracy of the voltage measurement itself is not so good.This is because the voltage measurement is affected by a voltage drop inan electric current path at the time of measurement by all means.Further, a contact portion between the terminal on the secondary batteryside and the terminal on the measuring instrument side varies to someextent every time they are connected to each other, so that a degree ofthe voltage drop also varies every measurement. In view of this, it isconceivable to improve measurement accuracy such that a measurement timeof a self-discharge amount is shortened by use of current measurementinstead of the voltage measurement. This is because the current isuniform at any part in a circuit and therefore the current measurementis barely affected by the contact portion, differently from the voltagemeasurement. Even so, successful determination may not be achievablejust by replacing the voltage measurement with the current measurement.This is because the measurement results are affected by variations invarious conditions such as a charging voltage or a measurementenvironment of the secondary battery.

The disclosure provides an inspection method of an electrical storagedevice and a manufacturing method of an electrical storage device eachof which can perform quality determination of the electrical storagedevice quickly regardless of variations in various conditions.

One aspect of the disclosure relates to an inspection method of anelectrical storage device and the inspection method includes: setting apseudo parasitic resistance value to be small in a case where a powerstorage capacity of the electrical storage device is large, as comparedto a case where the power storage capacity is small, while setting thepseudo parasitic resistance value to be large in the case where thepower storage capacity of the electrical storage device is small, ascompared to the case where the power storage capacity is large (a pseudoparasitic resistance setting step), the pseudo parasitic resistancevalue being obtained by dividing, by a current value of a circuit, anexcessive amount of a voltage value of an external power source withrespect to an initial voltage value of the electrical storage device; ina state where the pseudo parasitic resistance value is set, acquiring acurrent value after convergence of a current flowing through the circuitsuch that the circuit is formed by connecting the external power sourceto the electrical storage device in a direction where a voltage of theexternal power source is applied to the electrical storage device (acurrent measurement step); and determining quality of the electricalstorage device based on the current value (a quality determinationstep).

In the inspection method of the aspect, the quality of the electricalstorage device is determined based on current measurement without usingvoltage measurement. That is, the current value after convergenceacquired in the current measurement step indicates a self-dischargecurrent in the electrical storage device. On this account, it ispossible to determine the quality of the electrical storage device basedon the magnitude of the current value after convergence. Since thecurrent measurement is more accurate than the voltage measurement, surerdetermination can be performed more quickly. Here, as a resistance valueof the circuit is smaller, the current easily flows, and a timenecessary for convergence of the current is shorter. That is, it ispossible to perform inspection in a short time. However, the currentmeasurement has low accuracy. Particularly, in a case where the powerstorage capacity of the electrical storage device is small, a lowmeasurement accuracy is exhibited markedly. In view of this, in thepresent aspect, a value obtained by dividing, by the current value ofthe circuit, the excessive amount of the voltage value of the externalpower source with respect to the initial voltage value of the electricalstorage device is referred to as a pseudo parasitic resistance value,and the pseudo parasitic resistance value is set appropriately. That is,the pseudo parasitic resistance value is set in accordance with thepower storage capacity of the electrical storage device. Hereby, thequality determination of the electrical storage device can be performedquickly regardless of variations of various conditions. Note that thepseudo parasitic resistance value is a sum of an original parasiticresistance of the circuit and a fictitious resistance obtained byconverting an increment of the voltage value of the power source into anegative resistance value. In a case where the pseudo parasiticresistance value is set by adjusting a fictitious resistance component,when the power storage capacity of the electrical storage device issmall, the voltage of the external power source increases gradually inthe current measurement step, and when the power storage capacity of theelectrical storage device is large, the voltage of the external powersource increases rapidly.

In the inspection method of the first aspect, a parallel stack may beformed by connecting a plurality of the electrical storage devices inparallel to each other. In the acquiring of the current value, thecircuit may be formed by connecting the external power source to theparallel stack that has been charged. In the setting of the pseudoparasitic resistance value, the pseudo parasitic resistance value may beset in accordance with the power storage capacity of the parallel stack.In the determining of the quality of the electrical storage device,quality of the parallel stack may be determined. With such aconfiguration, it is possible to determine the quality of the wholeparallel stack more quickly.

A second aspect of the disclosure relates to a manufacturing method ofan electrical storage device, and the manufacturing method includes:performing initial charge such that an electrical storage device beingassembled and uncharged is charged to a predetermined charged state,such that a charged electrical storage device is formed (an initialcharge step); and inspecting the electrical storage device that has beencharged by the inspection method according to the first aspect (aninspection step).

With this configuration, an inspection method of an electrical storagedevice and a manufacturing method of an electrical storage device eachof which can perform quality determination of the electrical storagedevice quickly regardless of variations of various conditions can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a circuit diagram illustrating a configuration of a circuitmanufactured to perform an inspection method for a secondary battery inan embodiment;

FIG. 2 is an outside drawing illustrating an example of the secondarybattery as an inspection target in the embodiment;

FIG. 3 is a graph illustrating changes of a voltage and a current withtime according to a basic principle of inspection;

FIG. 4 is a graph illustrating an example of transition of a circuitcurrent when an output voltage is uniform;

FIG. 5 is a graph illustrating an example of transition of the circuitcurrent when the output voltage is increased;

FIG. 6 is a schematic view illustrating a state where a plurality ofsecondary batteries as inspection targets and spacers are bundled by abundling member so as to form a bundled body in the embodiment;

FIG. 7 is a circuit diagram in which a fictitious resistance isintroduced;

FIG. 8A is a graph illustrating fluctuations of a circuit current (in acase where a power storage capacity is small and a pseudo parasiticresistance is small);

FIG. 8B is a graph illustrating the fluctuations of the circuit current(in the case where the power storage capacity is small and the pseudoparasitic resistance is small) and is a graph illustrating a rangeindicated by a circle of an alternate long and short dash line in thegraph of FIG. 8A in an enlarged manner;

FIG. 9A is a graph illustrating fluctuations of a circuit current (in acase where a power storage capacity is large and a pseudo parasiticresistance is small);

FIG. 9B is a graph illustrating the fluctuations of the circuit current(in the case where the power storage capacity is large and the pseudoparasitic resistance is small) and is a graph illustrating a rangeindicated by a circle of an alternate long and short dash line in thegraph of FIG. 9A in an enlarged manner;

FIG. 10A is a graph illustrating fluctuations of a circuit current (in acase where a power storage capacity is small and a pseudo parasiticresistance is large);

FIG. 10B is a graph illustrating the fluctuations of the circuit current(in the case where the power storage capacity is small and the pseudoparasitic resistance is large) and is a graph illustrating a rangeindicated by a circle of an alternate long and short dash line in thegraph of FIG. 10A in an enlarged manner;

FIG. 11 is a graph qualitatively illustrating a relationship betweenpower storage capacity and set pseudo parasitic resistance; and

FIG. 12 is a schematic view illustrating a parallel stack of a pluralityof secondary batteries.

DETAILED DESCRIPTION OF EMBODIMENTS

The following describes an embodiment for embodying the disclosure indetail with reference to the attached drawings. As illustrated in FIG.1, an inspection method of an electrical storage device in the presentembodiment is performed in such a state that a circuit 3 is formed byconnecting a measuring device 2 to a secondary battery 1 that is anelectrical storage device as an inspection target. First described is abasic principle of the inspection method of the secondary battery 1 bythe measuring device 2.

Basic Principle

The secondary battery 1 is schematically illustrated in FIG. 1, butactually has a flat-square-shaped external appearance as illustrated inFIG. 2, for example. The secondary battery 1 in FIG. 2 is configuredsuch that an electrode laminated body 20 is provided in an outerpackaging body 10. The electrode laminated body 20 is configured suchthat a positive plate and a negative plate are laminated via aseparator. An electrolytic solution is also accommodated inside theouter packaging body 10 as well as the electrode laminated body 20.Further, positive and negative terminals 50, 60 are provided on anexternal surface of the secondary battery 1. Note that the secondarybattery 1 is not limited to the one with a flat-square shape asillustrated in FIG. 2 and may be one with other shapes such as acylindrical shape.

FIG. 1 will be further described. In FIG. 1, the secondary battery 1 isillustrated schematically. The secondary battery 1 in FIG. 1 isexpressed as a model constituted by an electromotive element E, aninternal resistance Rs, and a short circuit resistance Rp. The internalresistance Rs is placed in series with the electromotive element E. Theshort circuit resistance Rp is a model of a conductive path to be formedby very small foreign metallic particles that might enter the electrodelaminated body 20, and the short circuit resistance Rp is placed inparallel with the electromotive element E.

Further, the measuring device 2 includes a direct-current power source4, an ammeter 5, a voltmeter 6, and probes 7, 8. The ammeter 5 is placedin series with the direct-current power source 4, and the voltmeter 6 isplaced in parallel with the direct-current power source 4. An outputvoltage VS of the direct-current power source 4 is variable. Thedirect-current power source 4 is used to apply the output voltage VS tothe secondary battery 1 as will be described later. The ammeter 5measures a current flowing in the circuit 3. The voltmeter 6 measures avoltage between the probes 7, 8. In FIG. 1, the probes 7, 8 of themeasuring device 2 are connected to the terminals 50, 60 of thesecondary battery 1, so that the circuit 3 is formed.

Further, the circuit 3 of FIG. 1 has a parasitic resistance Rx. Theparasitic resistance Rx includes contact resistances between the probes7, 8 and the terminals 50, 60 in addition to lead wire resistances ofvarious parts in the measuring device 2. Note that, in FIG. 1, theparasitic resistance Rx is present only in the lead wire on the probe 7side, but this is just for convenience of drawing. In practice, theparasitic resistance Rx is present over the whole circuit 3.

In the inspection method by the measuring device 2, the magnitude of theself-discharge amount of the secondary battery 1 is inspected. When theself-discharge amount is large, the secondary battery 1 is defective,and when the self-discharge amount is small, the secondary battery 1 isnon-defective. For this purpose, the secondary battery 1 is chargedfirst before it is connected to the circuit 3. The secondary battery 1thus charged is connected to the circuit 3 and the self-discharge amountof the secondary battery 1 is calculated by the measuring device 2 inthat state. Based on the calculation result, the quality of thesecondary battery 1 is determined.

More specifically, the secondary battery 1 that has been charged isconnected to the circuit 3. At this time, the charged secondary battery1 to be connected to the circuit 3 is in a state where high-temperatureaging that is generally performed after charging is also finished and abattery voltage is stabilized. Note that the inspection itself in thepresent embodiment is performed at room temperature. Then, a batteryvoltage VB of the secondary battery 1 after charging andhigh-temperature aging is measured. A value measured herein is aninitial battery voltage VB1. Subsequently, the output voltage VS of themeasuring device 2 is adjusted so as to become the initial batteryvoltage VB1. Then, the secondary battery 1 is connected to the circuit3. The output voltage VS at this time is the same as the initial batteryvoltage VB1 of the secondary battery 1.

In this state, the output voltage VS is the same as the initial batteryvoltage VB1, and the output voltage VS and the battery voltage VB of thesecondary battery 1 are reverse to each other. On this account, thosevoltages are cancelled, so that a circuit current IB of the circuit 3 iszero. In this state, the output voltage VS of the measuring device 2 ismaintained to be constant at the initial battery voltage VB1 and is leftto stand.

A subsequent state of the circuit 3 is illustrated in FIG. 3. In FIG. 3,the horizontal axis indicates time, the vertical axis on the left sideindicates voltage, and the vertical axis on the right side indicatescurrent. In terms of the time on the horizontal axis, time T1 at theleft end in FIG. 3 indicates a timing at which application of the outputvoltage VS equal to the initial battery voltage VB1 is started asdescribed above. After time T1, the battery voltage VB graduallydecreases from the initial battery voltage VB1 due to self-discharge ofthe secondary battery 1. As a result, the balance between the outputvoltage VS and the battery voltage VB is lost, so that the circuitcurrent IB flows through the circuit 3. Hereby, the circuit current IBgradually increases from zero. The circuit current IB is directlymeasured by the ammeter 5. When time T2 comes after time T1, thedecrease of the battery voltage VB and the increase of the circuitcurrent IB are both saturated, so that the battery voltage VB and thecircuit current IB both become constant (VB2, IBs) afterward.

Note that, as apparent from FIG. 3, in comparison with a non-defectivesecondary battery 1, in a defective secondary battery 1, the increase ofthe circuit current IB and the decrease of the battery voltage VB areboth steep. Accordingly, a circuit current IBs after convergence in thedefective secondary battery 1 is larger than a circuit current IBs afterconvergence in the non-defective secondary battery 1. Further, a batteryvoltage VB2 after convergence in the defective secondary battery 1 islower than a battery voltage VB2 after convergence in the non-defectivesecondary battery 1.

The reason why the state of the circuit 3 after time T1 becomes thestate in FIG. 3 will be described. First, the battery voltage VBdecreases because of self-discharge of the secondary battery 1 asdescribed above. Due to the self-discharge, a self-discharge current IDflows through the electromotive element E of the secondary battery 1.When the self-discharge amount of the secondary battery 1 is large, theself-discharge current ID is large, and when the self-discharge amountis small, the self-discharge current ID is small. In the secondarybattery 1 in which the value of the short circuit resistance Rp issmall, the self-discharge current ID tends to be large.

In the meantime, the circuit current IB flowing due to the decrease ofthe battery voltage VB after time T1 is a current in a direction tocharge the secondary battery 1. That is, the circuit current IB works ina direction to restrain self-discharge of the secondary battery 1, andinside the secondary battery 1, the circuit current IB is reverse to theself-discharge current ID. When the circuit current IB increases andreaches the same magnitude as the self-discharge current ID, theself-discharge stops substantially. This happens at time T2. Hereby,after time T2, the battery voltage VB and the circuit current IB areboth constant (VB2, IBs). Note that whether the circuit current IB hasconverged or not should be determined by a known technique. For example,the value of the circuit current IB is sampled at suitable frequencies,and when a change of the value becomes smaller than a reference set inadvance, it is determined that the circuit current IB converges.

Here, as described above, the circuit current IB can be directly graspedas the reading value of the ammeter 5. In view of this, when a referencevalue IK is set to the circuit current IBs after convergence, qualitydetermination of the secondary battery 1 can be performed. When thecircuit current IBs after convergence is larger than the reference valueIK, the secondary battery 1 is defective with a large self-dischargeamount. In the meantime, when the circuit current IBs is smaller thanthe reference value IK, the secondary battery 1 is a non-defectiveproduct with a low self-discharge amount.

A necessary processing time (time T1→time T2) in such an inspectionmethod is shorter than the leaving time in the technique described inDescription of Related Art. Further, since the inspection method iscurrent measurement, the determination accuracy is higher. Note thatquality determination based on the battery voltage VB2 after convergencein FIG. 3 is not so preferable means. This is because the batteryvoltage VB does not necessarily appear accurately as the reading valueof the voltmeter 6. This is the basic principle of the inspection methodof the secondary battery 1 by the measuring device 2. Further, at thetime of manufacturing the secondary battery 1, an initial charge step ofinitially charging an assembled uncharged secondary battery 1 to apredetermined charged state so as to form a charged secondary battery 1,and an inspection step of inspecting the secondary battery 1 thuscharged can be performed. In the inspection step, the inspection methodshould be performed.

In the description so far, the output voltage VS of the measuring device2 is constant. However, the output voltage VS may not necessarily beconstant. When the output voltage VS is changed appropriately, it ispossible to further shorten the necessary processing time fordetermination. This will be described below.

An advantage obtained by changing the output voltage VS will bedescribed with reference to FIGS. 4 and 5. FIG. 4 illustrates an exampleof transition of an actual circuit current IB when the output voltage VSis constant as described above. In the example of FIG. 4, the outputvoltage VS is constant at a value initially determined and theconvergence (time T2) of the circuit current IB takes about 1.5 days.The example of FIG. 4 is a measurement example under the followingconditions in a state where a plurality of secondary batteries 1 andspacers 160 are bundled by a bundling member 130 so as to form a bundledbody 100 as illustrated in FIG. 6. The secondary batteries 1 in thebundled body 100 are pressurized in their thickness direction.

-   -   Battery capacity: 4 Ah    -   Positive-electrode active material: ternary system lithium        compound    -   Negative-electrode active material: graphite    -   Environmental temperature: 25° C.    -   Restraint load: 1 MPa

Although even the necessary processing time of 1.5 days in FIG. 4 issufficiently short as compared with determination based on the voltagemeasurement, the necessary processing time can be further shortened bychanging the output voltage VS. FIG. 5 is its example. In the example ofFIG. 5, the output voltage VS is increased, so that the circuit currentIB converges in only 0.1 days. Note that the example in FIG. 5 alsoemploys the same measurement conditions as FIG. 4. However, due toindividual differences of the secondary batteries 1 as the measurementtargets, the example in FIG. 5 does not have the same initial value ofthe output voltage VS and the same circuit current IB (IBs) afterconvergence as those in FIG. 4. Further, the measurement example in FIG.5 exemplifies a non-defective secondary battery 1, and in a case of adefective secondary battery 1, the circuit current IB (IB s) afterconvergence becomes a further large value.

The following further describes a case where the output voltage VS isincreased like FIG. 5. First, the circuit current IB of the circuit 3 inFIG. 1 is given by Equation (1) as follows based on the output voltageVS of the measuring device 2, the battery voltage VB, and the parasiticresistance Rx.IB=(VS−VB)/Rx  (1)

Here, when it is assumed that the output voltage VS is constant, thecircuit current IB increases due to a decrease of the battery voltage VBalong with self-discharge of the secondary battery 1, as has beendescribed in the foregoing. When the circuit current IB increases andreaches the same magnitude as the self-discharge current ID, thedischarge of the secondary battery 1 stops substantially. Hereby, asdescribed above, the battery voltage VB and the circuit current IB areboth constant (VB2, IBs) afterward. That is, the circuit current IBsafter convergence indicates the self-discharge current ID of theelectromotive element E of the secondary battery 1.

Even in the case where the output voltage VS is increased, Equation (1)is also established. However, since the output voltage VS increases, theincrease of the circuit current IB is faster by just that much than acase where the output voltage VS is constant. Because of this, anecessary time until the circuit current IB reaches the self-dischargecurrent ID is short. This is the reason why the circuit current IBconverges early as has been described in the foregoing. However, if theoutput voltage VS is increased blindly, the increase might be excessive.This results in that the circuit current IB does not convergeappropriately and the determination cannot be performed. On thataccount, it is necessary to restrict the degree of the increase of theoutput voltage VS. In the present embodiment, more specifically, theoutput voltage VS is increased within a range where the parasiticresistance Rx seems to become small in Equation (1). The reason is asfollows: when the parasitic resistance Rx becomes small, the circuitcurrent IB is increased by just that much.

In view of this, in the present embodiment, as illustrated in FIG. 7, aconcept of a fictitious resistance Rim is introduced. The fictitiousresistance Rim is a virtual resistance having a negative resistancevalue or a resistance value of zero. In the circuit diagram of FIG. 7,the fictitious resistance Rim is inserted in series with the parasiticresistance Rx. Such a resistance does not really exist, but theexamination is made such that a state where the output voltage VSincreases is replaced with a model in which the output voltage VS isconstant and an absolute value of a resistance value of the fictitiousresistance Rim increases instead. Note that the sum of the parasiticresistance Rx and the fictitious resistance Rim decreases but should bepositive. In the following description, the sum of the parasiticresistance Rx and the fictitious resistance Rim is referred to as apseudo parasitic resistance Ry. A circuit current in a model into whichthe pseudo parasitic resistance Ry is introduced is expressed byEquation (2) as follows.IB=(VS−VB)/(Rx+Rim)  (2)

Here, it is assumed that the parasitic resistance Rx is 5Ω. Then, a casewhere the fictitious resistance Rim is 0Ω and a case where thefictitious resistance Rim is −4Ω have different circuit currents IB.That is, the circuit current IB of the case of −4Ω (corresponding to thetime after the measurement is started) is five times larger than thecircuit current IB in the case of 0Ω (corresponding to the time when themeasurement is started) according to Equation (2). This is because thepseudo parasitic resistance Ry (=Rx+Rim) in the case of −4Ω is one-fifthof that in the case of 0Ω.

When Equation (2) is arranged, Equation (3) is obtained.VS=VB+(Rx+Rim)*IB  (3)

Equation (3) shows that, when the product of the pseudo parasiticresistance Ry with the circuit current IB is added to the batteryvoltage VB, the output voltage VS is obtained. Since the fictitiousresistance Rim in the pseudo parasitic resistance Ry does not actuallyexist as has been described in the foregoing, Equation (3) isestablished by increasing the output voltage VS to a voltage obtained byadding the product of the parasitic resistance Rx with the circuitcurrent IB to the battery voltage VB. That is, a value obtained bydividing an increment of the output voltage VS by the circuit current IBcorresponds to the absolute value of the fictitious resistance Rim.

In a case where the output voltage VS is adjusted to the initial batteryvoltage VB1 and the measurement is started as described above, theoutput voltage VS is increased in accordance with the circuit current IBat every timing at an appropriate frequency according to Equation (3).The frequency at which the output voltage VS is increased is about onetime per one second, for example. Note that the frequency may not befixed. With such a configuration, as the circuit current IB afterinspection is started largely increases, an increase width of the outputvoltage VS is larger. Further, when the increase of the circuit currentIB converges, the increase of the output voltage VS also converges.Hereby, the measurement as illustrated in FIG. 5 can be achieved.

Note that, according to the above description, the increase width of theoutput voltage VS with respect to an increment of the circuit current IBis the product of the parasitic resistance Rx with the circuit currentIB. That is, when the increase width of the output voltage VS isindicated by ΔVS, the increase width ΔVS is given as Equation (4) asfollows.ΔVS=Rx*IB  (4)

Alternatively, the increase width ΔVS may be a value obtained bymultiplying the product in Equation (4) by a positive coefficient K lessthan 1. A specific value of the coefficient K is any value within theabove range and should be determined in advance. That is, the increasewidth ΔVS may be calculated according to Equation (5) as follows.ΔVS=K*Rx*IB  (5)

Note that the product of the coefficient K with the parasitic resistanceRx may be found in advance as a constant M and the increase width ΔVS ofthe output voltage VS may be calculated by multiplying the circuitcurrent IB by the constant M. In this case, the output voltage VS in themiddle of the inspection is calculated according to Equation (6).VS=VB+M*IB  (6)

From the viewpoint that the increase of the circuit current IB convergesearly, it is the most effective to take the product of Equation (4) asthe increase width of the output voltage VS without any change. However,in this case, the pseudo parasitic resistance Ry might become negativedue to the accuracy of the value of the parasitic resistance Rx andother reasons. This results in that the change of the circuit current IBdiverges, so that necessary measurement cannot be performed. In view ofthis, the multiplication by the coefficient makes it possible to avoidthe risk of divergence.

Here, it is necessary to know the value of the parasitic resistance Rxin order to actually perform measurement by this control. Respectivecontact resistances between the probe 7 and the terminal 50 and betweenthe probe 8 and the terminal 60 in the parasitic resistance Rx changeevery time the circuit 3 is assembled. However, the parasitic resistanceRx including the contact resistances can be measured in the followingmanner, for example. That is, in FIG. 1, a reading value of thevoltmeter 6 is measured in two states, i.e., a state where the outputvoltage VS of the direct-current power source 4 is turned off and theterminals of the measuring device 2 are connected via a suitableresistance and a state where the terminals are disconnected. As aresult, the parasitic resistance Rx can be calculated based on aresistance value of the suitable resistance and two reading values ofthe voltmeter 6.

As such, the output voltage VS is increased while the value of thecircuit current IB is fed back to the output voltage VS. Hereby, theincrease of the circuit current IB can converge early. Thus, it ispossible to further shorten the necessary processing time fordetermination.

In the present embodiment, adjustment based on the power storagecapacity of the secondary battery 1 is further taken into considerationin addition to the above. This is because how much the pseudo parasiticresistance Ry can be reduced varies in accordance with the magnitude ofthe power storage capacity of the secondary battery 1. The reason is asfollows. In the circuit 3, the circuit current IBs after convergence ina case where the output voltage VS is constant is proportional to thepower storage capacity of the secondary battery 1. Meanwhile, a decreasewidth from the initial battery voltage VB1 of the battery voltage VB2after convergence is irrelevant to the power storage capacity of thesecondary battery 1. When this is applied to Equation (2), thedenominator (=the pseudo parasitic resistance Ry) in the right sidebecomes smaller as the power storage capacity is larger. This is thereason why a possible lower limit of the pseudo parasitic resistance Rydepends on the power storage capacity.

Further, this can be also described based on a relationship with theaccuracy of the voltage. When setting accuracy of the output voltage VSof the direct-current power source 4 is low, a deviation from a targetvalue for the value of the circuit current IB occurs. Assume a casewhere the output voltage VS to be applied temporarily becomes largerthan its target value and decreases to the target value after that. Thedeviation of the circuit current IB in this case is proportional to thedeviation of the battery voltage VB and inversely proportional to thepseudo parasitic resistance Ry. The deviation of the battery voltage VBdepends on how much the secondary battery 1 is charged while the outputvoltage VS larger than the target value is applied temporarily. However,it can be also said that the deviation of the battery voltage VB isinversely proportional to the power storage capacity of the secondarybattery 1. That is, when the power storage capacity of the secondarybattery 1 is large, the deviation of the battery voltage VB and thedeviation of the circuit current IB are small.

From the above description, it is found that, in a case where the powerstorage capacity of the secondary battery 1 is large, even if the pseudoparasitic resistance Ry is largely reduced, the divergence of thecircuit current IB can hardly occur. In the meantime, in a case wherethe power storage capacity of the secondary battery 1 is small, thedivergence of the circuit current IB due to reduction of the pseudoparasitic resistance Ry easily occurs. This is another reason why apossible lower limit of the pseudo parasitic resistance Ry depends onthe power storage capacity.

In view of this, in the present embodiment, the inspection is performedin the following procedure.

1. The circuit 3 illustrated in FIG. 1 (FIG. 7) is assembled such thatthe secondary battery 1 is connected to the measuring device 2.

↓

2. An initial value of the circuit current IB is set.

↓

3. The pseudo parasitic resistance Ry is set in accordance with thepower storage capacity of the secondary battery 1.

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4. The self-discharge current ID is measured by use of the pseudoparasitic resistance Ry thus set.

In the secondary battery 1 at the time of “1,” it is preferable that notonly initial charge but also high-temperature aging be finished, asdescribed above. As described above, “2” is to adjust the output voltageVS so that the circuit current IB becomes zero. Here, “3” will bedescribed later. Further, “4” is measurement of a convergence state ofthe circuit current IB illustrated in FIGS. 3 to 5. Using the pseudoparasitic resistance Ry means that the measurement is performed whilethe output voltage VS is increased, as described in FIG. 5.

Now, “3” will be described. Based on the foregoing, the pseudo parasiticresistance Ry is set in accordance with the power storage capacity ofthe secondary battery 1 based on the following thought. That is, whenthe power storage capacity is large, the pseudo parasitic resistance Ryis set to a small value. Note that the pseudo parasitic resistance Ryshould be a positive value as has been described in the foregoing. Inthe meantime, when the power storage capacity is small, the pseudoparasitic resistance Ry is set to a large value. Here, as the powerstorage capacity of the secondary battery 1, a designed value may beused without any change. Note that an actual value may be used.Measurement of the actual value of the power storage capacity may beperformed such that the secondary battery 1 that is fully charged isdischarged at a uniform discharging current and a necessary time to takeuntil the secondary battery 1 is fully discharged is measured.

Accordingly, in a current measurement step in “4,” the self-dischargecurrent ID is measured by use of the pseudo parasitic resistance Ry setin “3.” Hereby, in a case where the power storage capacity is small, thedegree of the increase of the output voltage VS is set moderately. Inthe meantime, in a case where the power storage capacity is large, thedegree of the increase of the output voltage VS is set to be high. Thismakes it possible to shorten the measurement time to its limit that doesnot cause the divergence of the circuit current IB.

Here, the deviation of the circuit current IB due to the power storagecapacity of the secondary battery 1 appears as a fluctuation of theactually measured circuit current IB. This will be described withreference to FIGS. 8A to 10B. The graphs in the figures each illustratechanges of the circuit current IBs in the circuit 3 with time. A partafter convergence is particularly illustrated on the lower side in anenlarged manner. Note that it should be noted that scales of verticalaxes are not the same.

Among three graphs, FIGS. 8A, 8B and FIGS. 9A, 9B are focused first.FIGS. 8A, 8B illustrate a graph in a case where the power storagecapacity is small (4 Ah), and FIGS. 9A, 9B illustrate a graph in a casewhere the power storage capacity is large (35 Ah). The pseudo parasiticresistance Ry is 0.1Ω (small) in either case. When the graph of FIG. 8Bis compared with the graph in FIG. 9B, they are different in amplitude Fof the circuit current IB. The amplitude F in FIG. 9B with a large powerstorage capacity is about less than 1 μA, whereas the amplitude F inFIG. 8B with a small power storage capacity is about 4 μA to 5 μA. Ofcourse, the magnitude relationship of the circuit current IB itselfcorresponds to the magnitude of the power storage capacity, but themagnitude of the amplitude F is reversed thereto. As such, the casewhere the power storage capacity is large is compared with the casewhere the power storage capacity is small, while their pseudo parasiticresistances Ry are set to the same. Here, it is found that the deviationof the circuit current IB is large in the case where the power storagecapacity is small. This indicates that the value of 0.1Ω as the pseudoparasitic resistance Ry is too small for the secondary battery 1 with asmall power storage capacity of 4 Ah.

Now, FIGS. 8A, 8B and FIGS. 10A, 10B are focused. FIGS. 8A, 8Billustrate a graph in a case where the pseudo parasitic resistance Ry isset to be small (0.1Ω), and FIGS. 10A, 10B illustrate a graph in a casewhere the pseudo parasitic resistance Ry is set to be large (1Ω). Thepower storage capacity is 4 Ah (small) in either case. The amplitude Fof the circuit current IB in the graph of FIG. 8B is compared with thatof the graph in FIG. 10B. The amplitude F in FIG. 10B with a largeresistance is about less than 0.5 μA, whereas the amplitude F in FIG. 8Bwith a small resistance is about 4 μA to 5 μA. As such, the case wherethe pseudo parasitic resistance Ry is large is compared with the casewhere the pseudo parasitic resistance Ry is small, while their powerstorage capacities are set to the same. Here, it is found that thedeviation of the circuit current IB is large in the case where thepseudo parasitic resistance Ry is small. This indicates that the valueof 0.1Ω as the pseudo parasitic resistance Ry is too small for thesecondary battery 1 with a small power storage capacity of 4 Ah.

Thus, from the comparison between FIGS. 8A, 8B and FIGS. 9A, 9B and thecomparison between FIGS. 8A, 8B and FIGS. 10A, 10B, it is found that, in“3,” in the case where the power storage capacity is large, the pseudoparasitic resistance Ry should be set to a small value, and in the casewhere the power storage capacity is small, the pseudo parasiticresistance Ry should be set to a large value. Note that FIGS. 8A, 8Billustrate an example (a comparative example) of unfavorable setting asunderstood from the above. In the graph of FIGS. 8A, 8B, the circuitcurrent IB does not diverge, but the circuit current IB may divergewithout converging depending on a setting degree. Further, a case wherethe power storage capacity and the pseudo parasitic resistance Ry areboth large is not illustrated, but, in this case, the amplitude F of thecircuit current IB is remarkably small while a convergence time of thecircuit current IB cannot be shortened so much.

Hereby, a qualitative relationship between the power storage capacity ofthe secondary battery 1 as an inspection target and the pseudo parasiticresistance Ry to be set is a relationship going downward toward theright side as illustrated in the graph of FIG. 11. Specific values ofthe vertical axis and the horizontal axis are not described in FIG. 11,but a value of the pseudo parasitic resistance Ry to each value of thepower storage capacity should be determined in advance based on a typeor a specification of the secondary battery 1 as an inspection target,required inspection accuracy, and the like.

Next will be described a case where a parallel stack of secondarybatteries 1 is taken as an inspection target. The inspection of thepresent embodiment can be performed on a single secondary battery 1 asan inspection target and can be also performed on a parallel stack 101as an inspection target. In the parallel stack 101, a plurality ofsecondary batteries 1 is connected in parallel to each other asillustrated in FIG. 12. The power storage capacity of the secondarybattery 1 in this case indicates a sum of individual secondary batteries1 included in the parallel stack 101. Further, the parallel stack 101may be the bundled body 100 by use of the bundling member 130 asillustrated in FIG. 6.

When the parallel stack 101 is taken as an inspection target, theinspection can be performed in a shorter time in comparison with a casewhere a single secondary battery 1 is taken as an inspection target.This is because the power storage capacity of the parallel stack 101 islarger than the power storage capacity of the single secondary battery1, naturally, and therefore, the inspection can be performed by settingthe pseudo parasitic resistance Ry to be lower. Note that thedetermination performable when the parallel stack 101 is inspected bythe method in the present embodiment is about the quality of the wholeparallel stack 101, and not about the quality of each of the secondarybatteries 1 included therein.

As specifically described above, with the present embodiment, thequality inspection of the secondary battery 1 is performed based on aconvergence value IBs of the circuit current IB. On this account, it ispossible to perform inspection in a short time with high accuracy, ascompared to a case where the inspection is performed based on voltagemeasurement. Further, by introducing the concept of a negativefictitious resistance Rim, that is, by increasing the output voltage VSof the measuring device 2, further shortening of the inspection time isachieved. Further, the pseudo parasitic resistance Ry as a sum of thefictitious resistance Rim and the parasitic resistance Rx is set inaccordance with the power storage capacity of the secondary battery 1(or the parallel stack 101) as an inspection target, so that a qualityinspection is performed by optimum setting suitable for the powerstorage capacity. Thus, an inspection method of an electrical storagedevice and a manufacturing method of an electrical storage device eachof which can perform quality determination of the electrical storagedevice quickly regardless of variations of various conditions areachieved.

Note that the present embodiment is merely an example and does not limitthe disclosure at all. Accordingly, it goes without saying that thedisclosure can be altered or modified variously within a range that doesnot deviate from the gist of the disclosure. For example, thedescription of the embodiment deals with an aspect in which the outputvoltage VS is increased from the same voltage as the initial batteryvoltage VB1. In the meantime, such an aspect is also employable that aninitial value VSI of the output voltage VS is daringly set to be higherthan the initial battery voltage VB1.

Further, in the embodiment, the pseudo parasitic resistance Ry is set byadjusting the fictitious resistance Rim, but the pseudo parasiticresistance Ry can be set by adjusting the parasitic resistance Rx.Further, the inspection method of the present embodiment is not limitedto a secondary battery just manufactured as a new product, and can beperformed on a used secondary battery for a remanufacturing process of aused assembled battery. Further, an electrical storage device as adetermination target is not limited to the secondary battery and may becapacitors such as an electric double layer capacitor and a lithium-ioncapacitor.

What is claimed is:
 1. An inspection method of an electrical storagedevice, the inspection method comprising: setting a pseudo parasiticresistance value to be small in a case where a power storage capacity ofthe electrical storage device is large, as compared to a case where thepower storage capacity is small, while setting the pseudo parasiticresistance value to be large in the case where the power storagecapacity of the electrical storage device is small, as compared to thecase where the power storage capacity is large, the pseudo parasiticresistance value being obtained by dividing, by a first current value ofa circuit, an excessive amount of a voltage value of an external powersource with respect to an initial voltage value of the electricalstorage device; in a state where the pseudo parasitic resistance valueis set, acquiring a second current value after convergence of a currentflowing through the circuit such that the circuit is formed byconnecting the external power source to the electrical storage device ina direction where a voltage of the external power source is applied tothe electrical storage device; and determining quality of the electricalstorage device based on the second current value.
 2. The inspectionmethod according to claim 1, wherein: a parallel stack is formed byconnecting a plurality of the electrical storage devices in parallel toeach other; in the acquiring of the second current value, the circuit isformed by connecting the external power source to the parallel stackthat has been charged; in the setting of the pseudo parasitic resistancevalue, the pseudo parasitic resistance value is set in accordance withthe power storage capacity of the parallel stack; and in the determiningof the quality of the electrical storage device, quality of the parallelstack is determined.
 3. A manufacturing method of the electrical storagedevice, the manufacturing method comprising: providing the electricalstorage device; connecting the external power source to the electricalstorage device; performing initial charge such that the electricalstorage device being assembled and uncharged is charged to apredetermined charged state, such that the electrical storage devicethat has been charged is formed; and inspecting the electrical storagedevice that has been charged by the inspection method according to claim1.